The invention described herein was made by an employee of the United States and may be manufactured and used by or for the Government for any governmental purpose without payment of any royalties thereon or therefor.
1. Field of the Invention
The present invention generally relates to the field of precision force measurement systems. In particular, a system is provided which facilitates the calibration of high precision multi-axis load cells, including wind tunnel force balances. An example of one wind tunnel force balance is described by U.S. Pat. No. 5,663,497, Six Component Wind Tunnel Balance, by Philip Mole.
Strain gauged force balances are used widely to measure forces applied to an object in a controlled test environment. These balances are commonly used to measure three components of aggregate force (axial, side, and normal, or vertical, forces; i.e. Fx, Fy, Fz), as well as three moments (roll, pitch, and yaw; i.e. Mx, My, Mz). The use of force balances has been widespread for many years, especially in the aerodynamic research industry, and includes applications in the estimation of important aerodynamic performance coefficients based on testing scaled aircraft models in wind tunnels.
Conventionally, force balances have been calibrated manually, using a complex system of free hanging precision weights, bell cranks, and/or other mechanical components. Conventional methods are generally accurate, but are often quite complex and labor-intensive, requiring three to four man-weeks to complete each full calibration. To ensure accuracy, gravity-based loading is typically utilized, however this often causes difficulty when applying loads in three simultaneous, orthogonal axes. A complex system of levers, cranks, and cables must be used, introducing increased sources of systematic error, and significantly increasing the time and labor intensity required to complete the calibration.
2. Description of the Related Art
In 1962, the National Aeronautics and Space Administration (NASA) acquired a semi-automatic method for calibrating force balances. Fully automated designs were then developed by Carl Schenk A G (1989) in Germany and Israel Aircraft Industries (1991) in Israel and subsequently have been made available in the United States. Utilizing these automated systems, combined with an abbreviated manual calibration, reduced the time involved in calibrating a balance to approximately two days, however the new designs still had significant disadvantages.
Each of these systems deteriorates the accuracy of the manual calibration system and is large, complex, and expensive. These calibration systems are not portable, and must be installed at a fixed location. In addition, the calibration system accuracy is difficult to experimentally verify. Since system accuracy is based on the combined accuracy of high precision load cells and position sensors, any complex load makes the resolution of force and moment vector orientation and magnitude relative to the coordinate system of the multi-axis load cell undergoing calibration extremely critical in overall system performance. Typically, automatic systems in the related art can only infer system accuracy of between 0.1% and 0.2% by comparing their calibrations of test balances with calibrations performed using traditional manual loads.
The Israeli system (U.S. Pat. No. 5,279,144 by Michael Levkowitch) is currently in commercial use in this country. Although this system substantially decreases the time required to perform a calibration, its cost of manufacture is very high, and the design introduces several additional sources of error to the calibration. Multiple load cells, force generators, and position sensors are used instead of free-swinging dead weights, so determination of the resultant force and moment vector magnitude and orientation relative to the coordinate system of the multi-axis load cell undergoing calibration is affected by increased potential sources of systematic error.
In 1996, NASA acquired patent rights to an automatic force balance calibration system, similar in many respects to the German (Schenk) design, which used a pre-calibrated reference balance to calibrate a test balance (see U.S. Pat. No. 5,533,380, Automatic Force Balance Calibration System, by Alice T. Ferris.). The design was portable and less expensive, but it did not improve calibration accuracy as compared to the manual calibration system. Questions evolved concerning the fundamental accuracy limits of calibrating one balance against another. The boundary conditions for the balance system differed significantly from those occurring when the balance is used in a wind tunnel. Also, the reference balance itself had to be calibrated by some other manual means. Accordingly, the need still existed for an accurate, efficient balance calibration system that is inexpensive to manufacture and inexpensive to use.
It is therefore an objective of the present invention to provide a calibration system which is inexpensive to manufacture, which requires minimal user time to set and operate, and which provides a high level of accuracy. Another objective of the present invention is to provide a calibration system which is a relatively small, simple system, and could conceivably be moved between sites and used to perform calibrations under varying environmental conditions.
In accordance with these and other objectives, which objectives will become obvious to one reading the text below, the present invention includes a simple, inexpensive, and highly accurate system for calibrating multi-axis load cells, including wind tunnel force balances. The system preferably allows for single vector calibration, meaning that single, calibrated dead-weight loads are applied essentially in only one direction to generate six component combinations of load relative to the multi-axis load cell coordinate system, thereby reducing the number of sources of inaccuracy related to load measurement, load application and balance positioning.
In the current best mode of operation, the system utilizes a load applied solely in the gravitational direction, in the form of calibrated weights hung from a load rod, although the system""s broader concept includes any measurable load applied in a single known direction, conceivably produced by actuators or other automated dead weight application devices. The calibration system can manipulate the multi-axis load cell being calibrated in three-dimensional space, while keeping the essentially uni-directional calibration load aligned in its single-vector position, whether that position be of the gravitational orientation or another prescribed direction.
At least one embodiment of the present invention utilizes a novel system of bearings and knife-edge rocker guides to maintain the load orientation, regardless of the variable three-dimensional orientation of the multi-axis load cell. An assembly of rotary tables with perpendicular axes of rotation can be used to manipulate the load cell in three-dimensional space. The three-dimensional manipulation of the multi-axis load cell allows the uni-directional load to be used to produce three force vectors on the balance (axial, side, and normal/vertical loads; i.e. Fx, Fy, and Fz), and three moment vectors (pitch, yaw, and roll moments, or My, Mz, Mx, respectively). The force and moment vectors can be produced on the multi-axis load cell without moving, replacing, or modifying the calibrated uni-directional load. As a result, the use of a single vector calibration load reduces the set-up time for the multi-axis load combinations needed to generate a complete calibration mathematical model.
The system also reduces load application inaccuracies caused by the conventional requirement to generate multiple force vectors. The simplicity of the system reduces calibration time and cost, while simultaneously increasing calibration accuracy. The system can be small enough to be conceivably used for on-site balance calibration in wind tunnel facilities, significantly decreasing the inconvenience, scheduling considerations, and costs associated with conventionally required dedicated calibration laboratories.
Additionally, the system can be adaptable to a variety of environmental calibration conditions, including thermal loading up to about two hundred fifty degrees Fahrenheit, cooling to about three hundred degrees below zero Fahrenheit, and pressurized calibration from about one to about fifteen hundred pounds per square inch. Also within the scope of the present invention, inter alia, is the use of dynamic calibration with time dependent loads, a fully automated system, a battery powered wireless data acquisition and control system that eliminates parasitic load paths created by electronic leads, a safety system which monitors balance signals to prevent unintentional overloads, and a magnet moment arm system used to apply co-linear force and moment vectors.
A software system can be used with the calibration hardware to record all signals, applied loads, and balance positions, to process data, and to supply the system user with resulting calibration results. The software system may also be used to provide computer-controlled automation of the calibration system.
A statistically optimal calibration design can also be used as part of the calibration methodology, to dictate the position of the applied load, the magnitude of the applied, the orientation of the multi-axis load cell, the order in which these settings are executed, and the time constraints in which these settings are performed.